Organ transplant rejection and the various autoimmune diseases are thought to be primarily the result of a T-cell mediated immune response. This T-cell mediated immune response is initially triggered by helper T-cells which are capable of recognizing specific antigens. These helper T-cells may be memory cells left over from a previous immune response or naive cells which are released by the thymus and may have any of an extremely wide variety of antigen receptors. When one of these helper T-cells recognizes an antigen present on the surface of an antigen presenting cell (APC) or a macrophage in the form of an antigen-MHC complex, the helper T-cell is stimulated by signals emanating from the antigen-specific T-cell receptor, co-receptors, and IL-1 secreted by the APC or macrophage, to produce IL-2. The helper T-cells then proliferate. Proliferation results in a large population of T-cells which are clonally selected to recognize a particular antigen. T-cell activation may also stimulate B-cell activation and nonspecific macrophage responses. Some of these proliferating cells differentiate into cytotoxic T-cells which destroy cells having the selected antigen. After the antigen is no longer present, the mature clonally selected cells will remain as memory helper and memory cytotoxic T-cells, which will circulate in the body and recognize the antigen should it show up again. If the antigen triggering this response is not a foreign antigen, but a self antigen, the result is autoimmune disease; if the antigen is an antigen from a transplanted organ, the result is graft rejection. Consequently, it is desirable to be able to regulate this T-cell mediated immune response.
CD45 antigen (CD45) is expressed on most leukocytes. Indeed, it was previously thought that a common CD45 antigen was present on all leukocytes, for which reason the receptor was originally known as the Leukocyte Common Antigen (LCA). Monoclonal antibodies to CD45 were proposed as a means of effectively eliminating all leukocytes where desirable, for example, purging an organ to be transplanted of passenger leukocytes prior to transplantation using nonspecific CD45 monoclonal antibody. See, e.g., WO 91/05568.
CD45 is a protein tyrosine phosphatase, and it participates in T-cell activation by dephosphorylating the protein tyrosine kinase P56lck at a tyrosine residue.
It has recently been shown that different isoforms of CD45 are generated by alternate splicing of a single primary transcript of the CD45 gene. These CD45 isoforms include CD45RA, CD45RB, and CD45RO. CD45RA contains the expression products of exons 4, 5, and 6 (sometimes referred to as A, B, and C) of the CD45 gene; CD45RB contains the expression products of exons 4 and 5, but not 6; CD45RO does not contain the expression products of any of the three exons 4, 5, or 6. See Hall et al, "Complete Exon-Intron Organization of the Human Leukocyte Common Antigen (CD4S) Gene", J. IMMUNOLOGY, Vol. 141, pp. 2781-2787 (1988), herein incorporated by reference and Streuli et al, "Characterization of CD451 and CD45R Monoclonal Antibodies Using Transfected Mouse Cell Lines that Express Individual Human Leukocyte Common Antigens", J. IMMUN., Vol. 141, pp. 3910-3914, No. 11 (1988), herein incorporated by reference. The significance of this variable expression, however, has been unclear.
It has now been discovered that different types of T-cells may predominantly express one or another of these CD45 isoforms of CD45 expression. Naive helper T-cells and memory T-cells express predominately CD45RA and CD45RO respectively. CD45RB expression is more variable, but it appears to be expressed on naive helper T-cells and especially on naive cells which have been stimulated to proliferate. CD45RB expression is down-regulated as the cells become activated. It has now been shown that compounds which suppress CD45RB expression but not CD45RO expression are capable of selectively inhibiting the T-cell mediated immune response without destroying the pool of memory T-cells. CD45RB is of particular interest because if CD45RB function is suppressed, the proliferating cells will fail to mature, and as a result the antigen specific clone they comprise will be deleted. Consequently, CD45RB suppressors have a great advantage over current immunosuppressants in that (i) they act on a particular T-cell population rather than having an overall immunosuppressive effect, thereby avoiding the risk of side effects associated with over-suppression of the immune system; and (ii) they are capable of conferring long term tolerance to a particular antigen when they are administered contemporaneously with exposure to antigen, e.g., just before and after an organ transplant or during an acute phase of an autoimmune disease.